Si photonic wire waveguides have a large contrast between refractive indexes of a core and its neighborhood. Such waveguides have enabled downsizing of optical devices. The typical cross-section of the Si photonic wire waveguides in the 1.55 μm band measures 220 nm thick by 450 nm wide. Highly sophisticated CMOS process enables it to mass-produce an optical integrated circuit that integrates many fine optoelectronic devices, thereby enabling it not only to prepare optical interconnection among devices or boards, but also to promise large-capacity optical interconnections for inter-chip or intra-chip.
Transmitting and receiving functions need at least to employ the Si photonic wire waveguides for optical interconnections. The application of the waveguides to optical interconnections for inter-chip or intra-chip requires downsizing, low power consumption (high efficiency), and high speed of devices. As for the receiver, waveguide-type Ge- or InGaAs-photodetectors with a length of 5 to 10 μm and a width of several μm are integrated on a Si photonic wire waveguide; and achieve an efficiency of about 1 mA/mW and a frequency band width of several GHz to several tens of GHz. As for the transceiver, it is very difficult to realize a highly-efficient laser with Si with an indirect band gap. Thus, it is common to combine a Si optical modulator with an external light source. The Si optical modulators include an electro-absorption modulator, a Mach-Zehnder modulator, and a ring modulator. The ring modulator is only one ultracompact modulator (its footprint is not more than 100 μm2) thereamong that can be applied to high-capacity optical interconnections.
A ring optical modulator has advantageous properties including compactness, a high efficiency, a high speed, and less return of reflected light. However, its resonance wavelength is very sensitive to the temperature (T), because Si has a positive thermo-optical coefficient, i.e, a positive temperature coefficient dn/dT, as high as +2×10−4/K. Mounting a temperature controller onto each ring optical modulator is not acceptable for the application to intra-chip optical interconnections, which has caused a major challenge to achieve an athermal ring modulator.
A passive optical ring filter can be athermalized by employing a polymer with a negative thermo-optical coefficient, i.e., a negative temperature coefficient dn/dT, as a clad material. Most of polymers have a thermo-optical coefficient of −1×10−4 to −3×10−4/K. Some polymers (polyurethane diacrylate: PUA) have a large negative thermo-optical coefficient of −4.5×10−4/K.
The refractive index of Si (3.48 at a wavelength of 1550 nm) is higher than that of polymers (1.4 to 1.6). A considerable amount of light should spread out into the polymer to make the temperature coefficient of the effective refractive index close to zero. In other words, it is required to employ a waveguide with a weak optical confinement. Such a waveguide has a cross-section of Si much smaller than that of the typical non-athermalized Si photonic wire waveguide. Decreasing the radius of the waveguide with a weak optical confinement causes a considerable increase in the radiation loss so that the downsizing has been difficult.
A Si ring optical modulator requires electric connections to inject current or to apply voltage. It is however difficult to provide the electric connections on or under a ring resonator, and is common to compose the ring resonator of a rib waveguide such that electrodes are formed on Si semiconductor slabs inside and outside of the ring resonator. In such a composition, the high refractive index of the Si slab outside the ring resonator causes a high radiation loss, thereby making it much difficult to reduce the radius of the waveguide.